Method for regulating a voltage and circuit therefor

ABSTRACT

A voltage regulator ( 10 ) having an undervoltage protection circuit  11  and a method for protecting against an output voltage out being less than a predetermined level. The voltage regulator has an undershoot limitation circuit ( 11 ) coupled between a feedback network ( 30 ) and a regulation section ( 42 ). A power factor correction circuit ( 46 ) is connected to the regulation section. An output voltage out from the power factor correction circuit ( 46 ) is fed back to the feedback network ( 30 ), which transmits a portion of the output voltage to the undershoot limitation circuit ( 11 ). If the output voltage is greater than the predetermined voltage level, a switching circuit portion ( 34 ) of the undershoot limitation circuit ( 11 ) transmits a normal control signal to the regulation circuit ( 42 ). If the output voltage is less than the predetermined voltage level, the switching circuit portion transmits an enhanced control signal to the regulation circuit. The enhanced control signal quickly brings the output voltage up to at least the minimum desired level.

FIELD OF THE INVENTION

This invention relates, in general, to power supplies and, more particularly, to power factor correction systems.

BACKGROUND OF THE INVENTION

Power Factor Correction (“PFC”) systems are typically used within power supply applications requiring AC/DC rectification. Rectifying arrangements for use in such an application may comprise a full wave voltage rectifier, typically in the form of a diode bridge, and a main Switch Mode Power Supply (“SMPS”) to provide regulation of the output waveform. PFC circuits are inserted between the line and the main SMPS to draw a sinusoidal current from the line and to provide Direct Current (“DC”) voltage to the main SMPS. For many systems to operate properly, it is desirable for the output voltage of the PFC circuit to be within a specified range. PFC circuits deliver a squared sinusoidal power that matches an average power demand of the load. Thus, when the power fed to the load is lower than the demand, the output capacitor compensates for the lack of energy by discharging and when the power fed to the load is greater than the demand, the capacitor stores the excess energy. As a consequence, a ripple appears in the output voltage that designers compensate for by integrating the output voltage. A drawback with the integration is that it degrades the dynamic performance of the PFC systems and makes them slow. For example, an abrupt decrease in the load results in high output voltage overshoot and an abrupt increase in the load results in a high output voltage undershoot.

Hence, there exists a need for a PFC system and a method of improving the dynamic performance and speed of the PFC system. In addition, it is desirable for the PFC system to be cost and time efficient to manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying drawing figures, in which like reference characters designate like elements, and in which:

FIG. 1 is schematic diagram of a voltage regulator having an undershoot limitation circuit in accordance with an embodiment of the present invention;

FIG. 2 is a schematic diagram of a voltage regulator having an undershoot limitation circuit and a start-up protection circuit in accordance with another embodiment of the present invention; and

FIG. 3 is a schematic diagram of a voltage regulator having an undershoot limitation circuit and a start-up protection circuit in accordance with yet another embodiment of the present invention.

DETAILED DESCRIPTION

Generally, the present invention provides a circuit and a method for limiting undervoltages and overvoltages in a regulated output signal. In particular, the circuit and method limit undershoot in the regulated output signal. In accordance with one embodiment, the present invention detects undershoot in an output signal such as, for example, a voltage signal, and transmits a boosted control signal to a regulation section of a regulator circuit. As those skilled in the art are aware, it is desirable for a voltage regulator to output a predetermined or nominal output voltage and for the output voltage to remain substantially constant independent of the load coupled to the regulator. The boosted control signal quickly adjusts the regulated output voltage to compensate for the undershoot. An advantage of a regulator of the present invention is that the original control signal is not modified. Rather, it is prevented from being transmitted to the regulation section. Another advantage is that the undershoot limitation circuit uses a transconductance amplifier to sample a feedback signal. The transconductance amplifier monitors the voltage of the feedback circuit without modifying it. Thus, it allows for a more accurate sampling of the feedback circuit.

FIG. 1 is a schematic diagram of a voltage regulator 10 comprising an undershoot limitation circuit 11 having an input 13 and an output 40, which output is connected to

an input 41 of a switching or Pulse Width Modulated (“PWM”) regulator 42. An output 44 of PWM regulator 42 is connected to an input 48 of a Power Factor Correction (“PFC”) circuit 46. An input 49 of PFC circuit 46 is connected to a rectifier 50 and an output 52 of PFC circuit 46 is connected to an input 54 of feedback network 30.

Feedback network 30 may be comprised of a pair of resistors 60 and 62 in which one terminal of resistor 60 is coupled for receiving a source of operating potential such as, for example, ground, and one terminal of resistor 62 serves as input 54 of feedback network 30. The other terminals of resistors 60 and 62 are connected together to form a node 66, which node 66 is connected to commonly connected inputs 16 and 24 of transconductance amplifier 12 and comparator 20, respectively. It should be understood that feedback network 30 is not limited to being a resistor divider network. Although feedback network 30 may be integrated in a semiconductor substrate along with undershoot limitation network 11 and PWM regulator 42, it is preferably provided as an off-chip circuit network.

In accordance with one embodiment, undershoot limitation circuit 11 includes an input section 15, which comprises a transconductance amplifier 12 and a comparator 20. Transconductance amplifier 12 has inputs 14 and 16 and an output 18, and comparator 20 has inputs 22 and 24 and an output 26. Inputs 16 and 24 of are connected to each other and inputs 14 and 22 are coupled for receiving reference voltages V_(REF1) and V_(REF2), respectively. Commonly connected inputs 16 and 24 are connected to feedback network 30. Output 18 of transconductance amplifier 12 is connected to an output 31 of a compensation network 32 and to an input 36 of a switching section or circuit 34, which comprises switches 70 and 74, a diode 72, and an inverter 76. Output 26 of comparator 20 is connected to an input 38 of switching section 34. Switch 70 has a terminal coupled to input 36 through diode 72 and a terminal connected to output 40 and to one terminal of switch 74. The terminal of switch 70 that is coupled to input 36 is also coupled to output 31 of compensation network 32 through diode 72. Input 38 is operatively coupled for controlling switch 74 and for controlling switch 70 through an inverter 76.

By way of example, compensation network 32 comprises a capacitor 80 coupled in a parallel configuration with a Zener diode 82.

PFC circuit 46 comprises an Insulated Gate Field Effect Transistor (“IGFET”) 84 having a gate that serves as input 48, a drain coupled to rectifier 50 through an inductor or coil 51, and a source coupled for receiving a source of operating potential such as, for example, ground. The drain of IGFET 84 is connected to one terminal of an inductor 51 and the other terminal of inductor 51 serves as input 49 of PFC circuit 46, which input is connected to voltage rectifier 50. PFC circuit 46 also includes a capacitor 86 having one terminal coupled to the cathode of diode 88 and a terminal coupled for receiving a source of operating potential such as, for example, ground. Thus, capacitor 86 is coupled to the drain of IGFET 84 through diode 88. The terminal of capacitor 86 connected to diode 88 and the cathode of diode 88 form an output or output node 52.

A load 90 is coupled in parallel with capacitor 86.

In operation, rectifier 50 provides a rectified voltage signal to input 85 of PFC circuit 46, which boosts the rectified voltage signal and generates an output voltage V_(OUT) at output 52. Output voltage V_(OUT) is fed back to feedback network 30 which generates a feedback signal V_(FB) at node 66. Transconductance amplifier 12 amplifies the difference between feedback signal V_(FB) and reference voltage V_(REF1) and generates an output current at output 18 that charges capacitor 80 of compensation network 32 if feedback voltage V_(FB) is less than reference voltage V_(REF1) or discharges capacitor 80 if feedback voltage V_(FB) is greater than reference voltage V_(REF1). The output current is a control signal. The voltage across capacitor 80 is a control voltage for controlling the power that is delivered to load 90 and is referred to as control voltage or control signal V_(CONTROL). The maximum voltage level that can appear across capacitor 80 is preferably limited by Zener diode 82, when it is active.

Comparator 20 compares feedback voltage V_(FB) with reference voltage V_(REF2) and generates a control signal at output 26 that is a logic low voltage level when feedback voltage V_(FB) is greater than reference voltage V_(REF2). The logic low voltage level at output 26 occurs when output voltage V_(OUT) is either in a nominal voltage condition or in an over-voltage condition. The logic low voltage level is transmitted to input 38 of switching circuit 34 causing switch 70 to close and switch 74 to open. This switch configuration applies or injects control voltage V_(CONTROL) into PWM regulator 42, which regulator 42 modulates or regulates the power delivered to load 90. By way of example, switches 70 and 74 are switching transistors. It should be noted that when a switching transistor is used as the switch, the switch is closed when the transistor is on and the switch is open when the transistor is off. As those skilled in the art are aware, a logic high voltage level control signal is also referred to as a true control signal and a logic low voltage level control signal is also referred to as a complementary control signal.

Comparator 20 generates a logic high voltage level at output 26 when feedback voltage V_(FB) is greater than reference voltage V_(REF2). Thus, control voltage V_(CONTROL) is applied to or injected into PWM regulator 42, which regulator 42 modulates or regulates the power delivered to load 90.

In response to output voltage V_(OUT) being an undervoltage or having an undershoot, feedback voltage V_(FB) is less than reference voltage V_(REF2). Comparator 20 compares voltage V_(FB) with voltage V_(REF2) and generates a logic high voltage level that appears at output 26. The logic high voltage level is transferred to input 38 of switching circuit 34 causing switch 70 to open and switch 74 to close. In addition, if the voltage appearing at output 18 becomes sufficiently high, Zener diode 82 becomes activated and clamps output 18 at a voltage V_(CONMAX). Because this switch configuration opens switch 70 and closes switch 74, it opens or breaks an electrical path between input 36 and output 40, thereby preventing control voltage V_(CONTROL) from appearing at input 41 of PWM regulator 42. This switch configuration also couples a control voltage or a control signal V_(CONMAX) to input 41 of PWM regulator 42. Control voltage V_(CONMAX) is at a higher voltage level than control voltage V_(CONTROL) so that it can force PWM regulator 42 and PFC circuit 46 to quickly increase output voltage V_(OUT). In accordance with one embodiment, V_(CONMAX) is the voltage across Zener diode 82 when it is at its Zener or breakdown voltage.

Referring now to FIG. 2, a schematic diagram of a voltage regulator 100 in accordance with another embodiment of the present invention is shown. Voltage regulator 100 comprises an undershoot limitation circuit 101 having an input 103 and an output 128, which is connected to an input 132 of a switching or Pulse Width Modulated (“PWM”) regulator 130. An output 134 of PWM regulator 130 is connected to an input 138 of a Power Factor Correction (“PFC”) circuit 136. An input 140 of PFC circuit 136 is connected to a rectifier 142 and an output 172 of PFC circuit 136 is connected to an input 146 of feedback network 120. Output 172 serves as the output of voltage regulator 100.

In accordance with one embodiment, feedback network 120 comprises a pair of resistors 150 and 152 connected such that one terminal of resistor 150 is coupled for receiving a source of operating potential such as, for example, ground and one terminal of resistor 152 serves as input 146 of feedback network 120. The other terminals of resistors 150 and 152 are commonly connected together to form a node 151, which node 151 is connected to input 104 of transconductance amplifier 102. Although feedback network 120 may be integrated in a semiconductor substrate along with undershoot limitation circuit 101, PWM regulator 130, and start-up circuit 162, it is preferably provided as an off-chip circuit network.

In accordance with one embodiment, undershoot limitation circuit 101 includes an input section 105 comprising a transconductance amplifier 102 having inputs 104 and 106 and an output 108, and a comparator 110 having inputs 112 and 114 and an output 116. Inputs 104 and 112 of transconductance amplifier 102 and comparator 110, respectively, are connected to each other and inputs 106 and 114 are coupled for receiving reference voltages V_(REF1) and V_(REF2), respectively. Reference voltage V_(REF2) is less than reference voltage V_(REF1). By way of example, reference voltage V_(REF2) is approximately ninety-five percent of reference voltage V_(REF1). Inputs 104 and 112 are connected to a feedback network 120. Transconductance amplifier 102 and comparator 110 are coupled to switching circuit 124. More particularly, output 108 of transconductance amplifier 102 is connected to a switch 122 and output 116 is connected to AND gate 160. It should be noted that the connection between output 116 and AND gate 160 is not shown for the sake of clarity. Rather, signal labels are used to denote the connection. Switch 122 has a terminal connected to output 108, a terminal connected for receiving an operating potential such as, for example, ground, and a control terminal operatively coupled for receiving a control voltage or control signal V_(OFF). Output 116 of comparator 110 is coupled for transmitting a control voltage or control signal V_(OUTL) to other parts of voltage regulator 100 such as, for example, switching circuit 124. Output 108 of transconductance amplifier 102 is connected to an input 126 of switching circuit 124. In addition, a capacitor 180 is coupled between output 108 and source of operating potential such as, for example, ground.

Switching circuit 124 comprises switches 156 and 158, wherein switch 156 has a terminal that serves as input 126 and a terminal connected to one terminal of switch 158. The commonly connected terminals of switches 156 and 158 serve as output 128 of switching circuit 124. The other terminal of switch 158 is coupled for receiving a reference voltage V_(UVP). Switching circuit 124 further includes a logic gate 160 having an input 161 coupled for receiving output signal V_(OUTL) from comparator 110 and an input 163 coupled for receiving a start-up indicator signal from a start-up indicator circuit 162. By way of example, logic gate 160 is an AND gate having inputs 161 and 163. The output of AND gate 160 is operatively coupled to switch 156 through an inverter 164 for controlling the state of switch 156. In addition, the output of logic gate 160 is operatively coupled to switch 158 for controlling the state of switch 158, i.e., controlling whether switches 156 and 158 are open or closed.

PFC circuit 136 comprises an Insulated Gate Field Effect Transistor (“IGFET”) 166 having a gate that serves as input 138, a drain coupled to rectifier 142 through an inductor or coil 169, and a source coupled for receiving a source of operating potential such as, for example, ground. The drain of IGFET 166 is also coupled to a capacitor 168 through a diode 170. More particularly, one terminal of capacitor 168 is connected to the cathode of diode 170 and the other terminal of capacitor 168 is coupled for receiving a source of operating potential such as, for example, ground. The commonly connected terminals of capacitor 168 and diode 170 form output 172 of voltage regulator 10, i.e., the cathode of diode 170 and the terminal of capacitor 168 that is not coupled for receiving the source of operating potential.

Start-up indicator circuit 162 comprises a latch 174 having an input S coupled for receiving control signal V_(OVL) through an inverter 176, a reset input R coupled for receiving control signal V_(OFF), and an output Q coupled to input 163 of two-input AND gate 160.

A load 178 is coupled in parallel with capacitor 168.

It is desirable for voltage regulator 100 to start operation in a manner that limits stress and audible noise during start up such as plugging in an appliance. In systems with a low regulation bandwidth, a capacitor such as capacitor 180 can be included to not only control the regulation bandwidth but also maintain smooth start-up characteristics. A control signal V_(OFF) is used to control the operational state of voltage regulator 100. When control signal V_(OFF) is at a logic high voltage level, voltage regulator 100 closes switch 122 and resets latch 174. Resetting latch 174 places a logic low voltage level at input 162 of AND gate 160 so that a logic low voltage level appears at its output. Control signal V_(OFF) can be generated when a power supply signal such as V_(CC) is too low, or if regulator 100 enters a shutdown mode, or if regulator 100 enters a hibernate, mode, or the like. The logic low voltage level at the output of AND gate 160 causes switch 156 to close and switch 158 to open. Thus, the voltage appearing at input 126 of switching circuit 124 is applied to or injected into PWM regulator 130, which regulator 130 modulates or regulates the power delivered to load 178. Closing switch 122 discharges capacitor 180.

When voltage regulator 100 begins operation, a very small amount of energy, if any, is transferred from PFC circuit 136 to feedback network 120. The size of capacitor 180 is selected so that it is slowly charged to promote a soft-start, i.e., it charges at a rate of less than approximately 100 volts per second. It should be understood that during start-up, voltage regulator 100 may be operating in an undershoot mode. This operating mode will be further described after describing the operation of undershoot limitation circuit 101.

Rectifier 142 provides a rectified voltage signal to input 140 of PFC circuit 136, which boosts the rectified voltage and generates an output voltage V_(OUT) at output 172. Output voltage V_(OUT) is fed back to feedback network 120 which generates a feedback signal V_(FB) at node 151. Transconductance amplifier 102 amplifies the difference between feedback signal V_(FB) and reference voltage V_(REF1) and generates an output current at output 108 that charges capacitor 180 if feedback voltage V_(FB) is less than reference voltage V_(REF1) or discharges capacitor 180 if feedback voltage V_(FB) is greater than reference voltage V_(REF1). The voltage across capacitor 180 is a control signal or voltage for controlling the power that is delivered to load 178 and is referred to as control voltage V_(CONTROL).

Comparator 110 compares feedback voltage V_(FB) with reference voltage V_(REF2) and generates an output voltage V_(OUTL) at output 116. Output voltage V_(OUTL) is at a logic low voltage level when feedback voltage V_(FB) is greater than reference voltage V_(REF2). The logic low voltage level at output 116 occurs when output voltage V_(OUT) is in either a nominal voltage condition or in an overvoltage condition. The logic low voltage level is transmitted to an input of switching circuit 124, which input is input 161 of two-input AND gate 160. Because V_(OUTL) is at a logic low voltage level, the output voltage of AND gate 160 is also at a logic low voltage level, which causes switch 156 to close and switch 158 to open. This switch configuration causes control voltage V_(CONTROL) to be applied to or injected into PWM regulator 130, which regulator 130 modulates or regulates the power delivered to load 178.

In response to output voltage V_(OUT) being less than a predetermined or nominal output level, i.e., voltage V_(OUT) being in an undershoot condition, feedback voltage V_(FB) is also less than reference voltage V_(REF2). Comparator 110 compares voltage V_(FB) with voltage V_(REF2) and generates a logic high voltage level that appears at output 116. The logic high voltage level is transmitted to input 161 of switching circuit 124, i.e., input 161 of two-input AND gate 160. Because voltage V_(OUTL) is at a logic high voltage level, the voltage level at the output of AND gate 160 is controlled by start-up indicator circuit 162, i.e., the voltage at output Q of latch 174. Due to the undershoot voltage condition, voltage V_(OVL) at the input of start-up indicator circuit 162 is at a logic low voltage level. The logic low voltage level is converted to a logic high voltage level by inverter 176 and transmitted to input 163. A logic high voltage level appears at the output of AND gate 160, resulting in switch 158 being closed and switch 156 being open. In this switch configuration, the voltage at output 108 of transconductance amplifier 102 is decoupled from output 128 and control voltage V_(UVP) is coupled to output 128. Control voltage V_(UVP) is at a higher voltage level than control voltage V_(CONTROL) so that it can force PWM regulator 130 and PFC circuit 136 to quickly increase output voltage V_(OUT).

Start-up indicator circuit 162 is included to allow voltage regulator 100 to bypass switching circuit 124 during start up at which time transconductance amplifier 102 charges capacitor 180. In addition, a control voltage V_(OVL) is set to a logic high voltage level. The logic high voltage level is transmitted to start-up indicator circuit 162 which inputs a logic low voltage level to input 163 of AND gate 160, which logic low voltage level appears at the output of AND gate 160, causing switch 156 to close and switch 158 to open. When capacitor 180 stops charging, control voltage V_(OVL) is set to a logic low voltage level indicating detection of the end of the start-up phase. The logic low voltage level is transmitted to start-up indicator circuit 162 which inputs a logic high voltage level to input 163 of AND gate 160. Placing a logic high voltage level on one of the inputs of AND gate 160 allows the voltage at the other input of AND gate 160 to control the output voltage level of AND gate 160. Thus, the voltage on input 163 of AND gate 160 controls whether control signal V_(CONTROL) or control signal V_(UVP) is transmitted to PWM regulator 130. It should be noted that control signal V_(OFF) discharges capacitor 180 by coupling it to ground when regulator 100 is off.

Referring now to FIG. 3, a schematic diagram of a voltage regulator 200 in accordance with another embodiment of the present invention is shown. Voltage regulator 200 comprises an undershoot limitation circuit 201 having an input 203 and an output 210. Like voltage regulator 100, voltage regulator 200 includes an input section 105, feedback network 120, PWM regulator 130, and PFC circuit 136. Rather than having switching circuit 124, voltage regulator 200 has a switching section 202. In accordance with one embodiment, switching section 202 comprises switches 204 and 206 and an inverter 207. One terminal of switch 204 serves as an input 208 and the other terminal of switch 204 is connected to a terminal of switch 206. The commonly connected terminals of switches 204 and 206 serve as output 210 of undershoot limitation circuit 201. The other terminal of switch 206 is coupled to a terminal of a switch 212, as well as to a terminal of a current source 214, and to a terminal of a capacitor 216. The other terminals of switch 212 and capacitor 216 are coupled for receiving a source of operating potential such as, for example, ground. A control voltage V_(OUTL) is operatively coupled to switch 204 through inverter 207 and to switch 206, i.e., it serves as a control signal for switches 204 and 206. Setting control voltage V_(OUTL) to a logic high voltage level opens switch 204 and closes switch 206 and setting control voltage V_(OUTL) to a logic low voltage level closes switch 204 and opens switch 206.

Unlike, voltage regulator 100, switch 122 is absent from voltage regulator 200.

In operation, rectifier 142 provides a rectified voltage signal to input 140 of PFC circuit 136, which boosts the rectified voltage and generates an output voltage V_(OUT) at output 172. Output voltage V_(OUT) is fed back to feedback network 120 which generates a feedback signal V_(FB) at node 151. Transconductance amplifier 102 amplifies the difference between feedback signal V_(FB) and reference voltage V_(REF1) and generates an output current at output 108 that charges capacitor 180 if feedback voltage V_(FB) is less than reference voltage V_(REF1) or discharges capacitor 180 if feedback voltage V_(FB) is greater than reference voltage V_(REF1). The voltage across capacitor 180 is a control signal or voltage for controlling the power that is delivered to load 178 and is referred to as control voltage V_(CONTROL).

Comparator 110 compares feedback voltage V_(FB) with reference voltage V_(REF2) and generates an output voltage V_(OUTL) at output 116. Output voltage V_(OUTL) is at a logic low voltage level when feedback voltage V_(FB) is greater than reference voltage V_(REF2). The logic low voltage level at output 116 occurs when output voltage V_(OUT) is in either a nominal voltage condition or in an overload condition. The logic low voltage level is transmitted to control input 209 of switching section 202, which controls the states of switches 204 and 206. Voltage V_(OUTL) being at a logic low voltage level causes switch 204 to be closed and switch 206 to be open. This switch configuration causes control voltage V_(CONTROL) to be applied to or injected into PWM regulator 132, which regulator 132 modulates or regulates the power delivered to load 178.

In response to output voltage V_(OUT) being lower than the nominal value, i.e., V_(OUT) being in an undershoot condition, feedback voltage V_(FB) is less than reference voltage V_(REF2). Comparator 110 compares voltage V_(FB) with voltage V_(REF2) and generates a logic high voltage level that appears at output 116. The logic high voltage level is transmitted to input 209. Voltage V_(OUTL) being at a logic high voltage level causes switch 204 to be open and switch 206 to be closed. In this switch configuration, the voltage at output 108 of transconductance amplifier 102 is decoupled from output 210 and control voltage V_(UVP1) is coupled to output 210.

Control voltage V_(UVP1) is the voltage across capacitor 216 and it is generated by current source 214 charging capacitor 216. Before start-up, control signal V_(OFF) was set to a logic high voltage level, thereby closing switch 212 and discharging capacitor 216. At or near start-up, control signal V_(OFF) is set to a logic low voltage level, which opens switch 212. Because the circuit is starting up, feedback signal V_(FB) is less than reference voltage V_(REF2), voltage V_(OUTL) is at a logic high voltage level, switch 204 is open, and switch 206 is closed. In this configuration current source 214 charges capacitor 216. An advantage of this embodiment is that during start-up, and as long as V_(FB) is less than V_(REF2), the voltage applied to PWM regulator 130 gradually increases which allows a soft start-up. Thus, the voltage applied to PWM regulator 130 is time varying.

By now it should be appreciated that methods and circuits for protecting against undershoot in an output voltage have been provided. In accordance with embodiments of the present invention, undershoot limitation circuits are included with the voltage regulator to increase the voltage at the input of the regulation portion. An advantage of the present invention is that the undershoot limitation circuits include switching circuits or networks that route a control voltage to the regulation portion during normal operation and an enhanced control voltage to the regulation portion when the output voltage of the voltage regulator experiences an undershoot condition. The enhanced control voltage or signal can be a time varying signal or a constant signal such as, for example, a logic signal. Another advantage of the present invention is that the undershoot limitation circuits include analog circuitry, i.e., a transconductance amplifier, that does not alter the feedback signal thereby improving the accuracy of the output voltage.

Although certain preferred embodiments and methods have been disclosed herein, it will be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods may be made without departing from the spirit and scope of the invention. For example, overshoot protection can be provided. In accordance with one embodiment related to regulator 10, the circuit can be modified to provide overshoot protection by switching the feedback signal (V_(FB)) to be at the non-inverting input of the comparator, applying a reference voltage V_(REF2) to the inverting input, and changing the control voltage V_(CONMAX) at the terminal of, for example, switch 74 to be a low voltage level. It is intended that the invention shall be limited only to the extent required by the appended claims and the rules and principles of applicable law. 

1. A method for regulating a voltage, comprising: comparing a feedback signal with a first reference signal; generating a first control signal in response to the comparison between the feedback signal and the first reference signal; passing a second control signal to a circuit in response to the first control signal; and passing a third control signal to the circuit in response to a complementary first control signal.
 2. The method of claim 1, further including comparing the feedback signal to the second reference signal.
 3. The method of claim 2, further including generating the second control signal in response to the comparison between the feedback signal and the second reference signal.
 4. The method of claim 3, wherein generating the second control signal includes using a transconductance amplifier to compare the feedback signal and to the second reference signal.
 5. The method of claim 2, wherein the first reference signal is lower than the second reference signal.
 6. The method of claim 1, wherein generating the first control signal in response to the comparison between the feedback signal and the first reference signal includes generating the first control signal in response to one of an undershoot voltage or an overload voltage.
 7. The method of claim 1, wherein the third control signal is at a higher voltage level than the second control signal.
 8. The method of claim 6, wherein the second control signal is one of a logic high signal or a logic low signal.
 9. The method of claim 1, wherein the third control signal is one of a time-varying signal or a constant signal.
 10. A method for regulating a voltage, comprising: comparing a feedback voltage to first and second reference voltages; generating at least one control signal in response to comparing the feedback voltage to the first reference voltage; generating a second control signal in response to comparing the feedback voltage to the second reference voltage; passing the second control signal to a circuit in response to the at least one control signal; and passing a third control signal to the circuit in response to at least one other control signal that is the complement of the at least one control signal.
 11. The method of claim 10, wherein generating the at least one control signal comprises generating a plurality of control signals.
 12. The method of claim 11, generating the plurality of control signals includes generating a true control signal and a complementary control signal.
 13. The method of claim 12, wherein generating at the plurality control signals in response to comparing the feedback voltage to the first reference voltage includes closing a first switch with the true control signal.
 14. The method of claim 13, wherein generating at the plurality control signals in response to comparing the feedback voltage to the first reference voltage includes opening a second switch with the complementary control signal.
 15. A voltage regulator, comprising: an undershoot limitation circuit, comprising: a transconductance amplifier having first and second inputs and an output, the first input coupled for receiving a feedback voltage and the second input coupled for receiving a first reference voltage; a comparator having first and second inputs and an output, the first input coupled for receiving the feedback voltage and the second input coupled for receiving a second reference voltage, the first reference voltage greater than the second reference voltage; and an switching circuit having first and second inputs and an output, the first input coupled to the output of the transconductance amplifier and the second input coupled to the output of the comparator; and a regulation circuit having an input and an output, the input coupled to the output of the undershoot limitation circuit.
 16. The voltage regulator of claim 15, wherein the switching circuit comprises: a first switch having first and second terminals, the first terminal coupled to the output of the transconductance amplifier; and a second switch having first and second terminals, the first terminal coupled to the second current carrying conductor of the first switch and the second terminal coupled for receiving a control signal.
 17. The voltage regulator of claim 16, further including a diode coupled between the first terminal of the first switch and the output of the transconductance amplifier.
 18. The voltage regulator of claim 16, further including an inverter coupled between the output of the comparator and a control terminal of the first switch.
 19. A method for regulating a voltage, comprising: providing first and second reference signals; comparing a feedback signal with the first reference signal; generating a first control signal in response to the comparison between the feedback signal and the first reference signal; generating a second control signal in response to the feedback signal being different from the second reference signal; and using the first and second control signals to control the operating state of an undershoot limitation circuit.
 20. The method of claim 19, wherein using the first and second control signals to control the operating state of an undershoot limitation circuit includes applying the second control signal to a latch.
 21. The method of claim 19, wherein using the first and second control signals to control the operating state of an undershoot limitation circuit includes using the second control signal to change the output state of the latch when the after a transconductance amplifier has stopped charging a control capacitor.
 22. A voltage regulator, comprising: an undervoltage protection circuit comprising: a comparator having first and second inputs and an output, the first input coupled for receiving a feedback voltage and the second input coupled for receiving a first reference voltage; a transconductance amplifier having first and second inputs and an output, the first input coupled for receiving the feedback voltage and the second input coupled for receiving a second reference voltage, the first reference voltage less than the second reference voltage; and a switching circuit having first, second, and third inputs and an output, the first input coupled to the output of the transconductance amplifier and the second input coupled to the output of the comparator; a regulation circuit having an input and an output, the input coupled to the output of the undervoltage protection circuit; and a start detection circuit having an input and an output, the input coupled for receiving a control signal and the output coupled to the third input of the undershoot protection circuit.
 23. The voltage regulator of claim 22, wherein the undervoltage protection circuit comprises: a logic circuit having first and second inputs and an output, the first input serving the first input of the voltage protection circuit, the second input serving as the third input of the voltage protection circuit; a buffer circuit having an input and an output, the input coupled to the output of the logic circuit; a first switch having first and second conductors, the first conductor coupled to the output of the comparator and the output of the buffer circuit operatively coupled to the first switch; and a second switch having first and second conductors, the first conductor coupled to the second conductor of the first switch, the second conductor coupled for receiving a source of potential, and the output of the logic circuit operatively coupled to the second switch.
 24. The voltage regulator of claim 23, wherein the buffer circuit is an inverter and the logic circuit is an AND gate.
 25. The voltage regulator of claim 24, wherein the start detection circuit comprises a latch having first and second inputs and an output, the first input coupled for receiving the second control signal, the second input coupled for receiving another control signal and the output coupled to the second input of the logic circuit.
 26. A voltage regulator, comprising: a first comparator having first and second inputs and an output, the first input coupled for receiving a feedback voltage and the second input coupled for receiving a first reference voltage; a second comparator having first and second inputs and an output, the first input coupled for receiving the feedback voltage and the second input coupled for receiving a second reference voltage, the first reference voltage less than the second reference voltage; a voltage protection circuit having first, second, and third inputs and an output, the first input coupled to the output of the first comparator and the second input coupled to the output of the second comparator; a regulation circuit having an input and an output, the input coupled to the output of the voltage protection circuit; a buffer circuit having an input and an output, the input coupled to the output of the first comparator; a first switch having first and second conductors, the first conductor coupled to the output of the second comparator and the output of the buffer circuit operatively coupled to the first switch; and a second switch having first and second conductors, the first conductor coupled to the second conductor of the first switch and the output of the first comparator operatively coupled to the second switch.
 27. The voltage regulator of claim 26, further including a third switch having a first conductor and a second conductor, the first conductor coupled to the second conductor of the second switch, the first conductor coupled for receiving a first source of operating potential, and a control signal operatively coupled to the switch.
 28. The voltage regulator of claim 27, further including a capacitor coupled between the second conductor of the second switch and the first source of operating potential.
 29. A method for regulating a voltage, comprising: providing first and second reference signals; comparing a feedback signal with the first reference signal; generating a first control signal in response to the comparison between the feedback signal and the first reference signal; generating a second control signal in response to the feedback signal being different from the second reference signal; and using the first control signal to control the operating state of an undershoot limitation circuit by operatively coupling the first control signal to a first switch and operatively coupling a complementary first control signal to a second switch.
 30. The method of claim 29, further including using the first control signal to close the first switch.
 31. The method of claim 29, further including: charging a charge storage device; and transmitting a voltage of the charge storage device through the first switch to a circuit.
 32. The method of claim 31, wherein the charge storage device is a capacitor and wherein charging the capacitor includes injecting a current through the capacitor. generating a third control signal that increases during a start-up phase of operation; and using the first and third control signals to control the operating state of an undershoot limitation circuit. 